Anchor strike

Also called Heatmap anchor, dominant strike

The heatmap cell with the most exposure of the chosen greek, either way, in the grid you're looking at.

On the By expiration heatmap, the anchor strike is the single cell, one strike in one expiration, holding the largest exposure of the chosen greek, positive or negative, across the strikes and expirations shown. Ties go to the strike nearer spot, then the earlier expiration. Its strike is underlined in orange on the strike axis and it's named in the legend.

Reading it

It answers "where is the heaviest positioning in this set?" in one look. It isn't the same as a wall: the call wall and put wall add every expiration at a strike together, while the anchor is one strike in one expiration, and it can be positive or negative. The heatmap's Net column shows that sum for every strike, and each expiration's own largest cell is printed in bold with a tick in its corner; the anchor is the largest of those.

Change the greek, the expiration set, or the strike range and the anchor can move, because it's chosen from the grid as it stands.

In Theta Vantage

The orange underline on a strike of the heatmap's By expiration view, with a thin orange outline on its cell.